In multiple channel wavelength division multiplexed (WDM) communication systems, optical amplifiers are used to boost signal powers to provide for longer transmission spans. When using optical amplifiers in such systems, it is important to balance the channels at the input to the amplifiers to assure that the available amplifier output power is shared equally among the channels. If the channels are not balanced across each amplifier input, the weaker signals reduce the transmission span distance.
Another problem faced is that optical amplifiers used in multiple channel wavelength division multiplexed communication systems often do not have uniform gain across all of the optical channels. This creates some channel imbalance. In other systems where optical amplifiers are used as repeaters, the cascade of the amplifiers results in signal inequalities even when they are well balanced at the head end.
Other problems relate to the configuration of a bi-directional line amplifier (BDLA) which uses a single amplifier. The receive signals from each direction needs to be balanced to assure proper gain and power sharing in the amplifier. Further, significant power level imbalances between channels at the end of a cascade of amplifiers may require attenuation of the strong channels to keep the signal level to the receiver at an acceptable level.
One conventional solution to balancing of channels has been to insert manually selected fixed attenuators or manually adjusted variable attenuators in each channel path before multiplexing them together. However, the selection or the adjustment of the attenuators requires manual measurement of each optical channel, since the source transmitter power level for each channel varies and is not precisely known beforehand. Furthermore, when the source transmit power changes, due to aging or module replacement, the attenuator adjustment process has to be repeated.